The Cr2+ reductions of the three complexes [(en)2Co(Met)]2+, [(en)2Co(MeCys)]2+, and [(en)2Co(Cys)]2+ (Met = methionine, MeCys = methylcysteine, Cys = cysteine) are described. The Co(III) complexes of methionine and methylcysteine are O,N-bonded chelates. Attack of Cr2+ occurs at the O atom in each of these complexes to produce the O-bonded monodentate Cr(III) product which slowly aquates. The kinetic data for the reduction of the methioninato complex are k25°C = (0.416 ± 0.006) M-1 s-1, μ = 1.0 M (LiClO4), ΔH‡ = 9.7 ± 0.9 kcal mol-1, and ΔH‡ = -28 ± 3 eu. The data for the corresponding methylcysteinato complex are k25°C = 0.56 ± 0.01 M-1 s-1, μ = 1.0 M (LiClO4), ΔH‡ = 9 ± 1 kcal mol-1, and ΔS‡ = -28 ± 4 eu. The [(en)2Co(Cys)]2+ complex is S,N bonded. Attack of Cr2+ occurs at the S atom to give the S-bonded monodentate intermediate which undergoes rapid ring closure, in the presence of excess Cr2+, to produce the S,O-bonded chelated product. If Cr2+ is in deficit, a series of reactions ultimately producing the S,N-bonded Cr(III) product also ensues. The rate of Cr2+ reduction of [(en)2Co(Cys)]2+ is (2.4 ± 0.3) × 105 M-1 s-1, μ = 1.0 M (LiClO4). The S-bonded monodentate intermediate is attacked by Cr2+ in both the protonated and unprotonated forms, as evidenced by an inverse hydrogen ion term in the rate law. The rate of reduction through the protonated form is (6 ± 3) × 103 M-1 s-1; through the unprotonated form, it is estimated to be (1.6 ± 0.4) × 107 M-1 s-1. Comparisons of these results to related systems are made and the results of the cysteine system are discussed in terms of the superexchange and resonance-transfer mechanisms.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry